1. Field of the Invention
The present invention relates to the encapsulation of thin workpieces, particularly electronic and microelectronic devices, to achieve a very thin, but void-free seal around the workpiece.
2. Description of Related Art
Microelectronic devices must be protected against moisture as well as assembly process and other environmental contaminants. This is commonly done by encapsulating the device in a mold compound, such as a thermosetting plastic, applied by a transfer molding process.
In a typical transfer molding machine used in the microelectronics industry, a thin electronic workpiece mounted on a lead frame is clamped between two halves of a split mold. The mold defines a mold cavity around the device with sufficient clearance to allow mold compound to be injected and flow around the device to encapsulate it. During the molding process mold compound is injected into an inlet and air inside the mold escapes from a vent.
The mold compound is initially provided in a non-liquid pellet form containing a desired quantity of the compound. The pellet is heated under pressure in a chamber until it is liquefied. A plunger then drives the liquefied mold compound into the mold cavity. The mold compound is allowed cure and the mold is opened, releasing the encapsulated microelectronic device.
Because smaller microelectronic devices are highly desirable, device manufacturers would like to reduce the thickness of the encapsulating layer of mold compound which encases each device. Thinner encapsulating layers also aid in improving device performance or reliability with regard to heat dissipation, resistance to coating damage under thermal stress and other parameters. However, as the distance between the inner mold surfaces and the electronic workpiece is decreased, it becomes more difficult to obtain a high quality void-free encapsulant around the entire device.
To obtain a void-free seal, the liquefied mold compound must enter the mold inlet and entirely fill the space in the mold cavity before the mold compound flow front arrives at the mold vent. If the mold compound reaches the vent before the mold is completely filled, an air bubble is trapped in the mold, creating a void.
To completely fill the mold cavity, the mold compound must flow between the upper mold surface and the upper surface of the device, between the lower mold surface and the lower surface of the device, and into the space surrounding the outer perimeter of the device. However, as the distance between the upper and lower mold surfaces and the device is reduced, so as to make the encapsulating coating thinner, it becomes more difficult for the mold compound to penetrate these regions.
If this distance is reduced too far, the mold compound will flow around the outer perimeter of the device before the mold compound flow front has displaced the air in the space above and below the device. The result is a void in the encapsulation material as an air bubble is pinched off in the center of the device.
As a result, transfer molding of semiconductor devices with conventional equipment has required that the distance from the inner mold surfaces to the device be at least about 200-250 micrometers. This ensures that there will be laminar flow of the molding compound into the mold and around the device. The exact minimum distance limit is, of course, a function of the specific mold compound used, the fillers it contains and process parameters, such as temperature, but, in general, reducing the distance from the inner mold surfaces to the device to less than some minimum distance results in unacceptable manufacturing losses due to the formation of voids.
Provided that sufficient clearance between the inner mold surfaces and the device is maintained, however, the flow of the mold compound during injection remains laminar, and the flow fronts above and below the device remain relatively balanced, so as to prevent the formation of voids. On the other hand, it is known that acceptable sealing of the device and protection against environmental contamination can be achieved with an encapsulation thickness that is well below this thickness limit.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of encapsulating an electronic workpiece with an outer coating of mold compound which is thinner than the thickness limit heretofore achievable with conventional transfer molding.
A further object of the invention is to provide a method of encapsulating an electronic workpiece with a coating of mold compound that is void-free.
It is yet another object of the present invention to provide a method of encapsulating an electronic workpiece by varying the mold dimensions while the mold compound is in a liquid state.
Yet another object of the present invention is to provide a mold for encapsulating an electronic workpiece that can vary in volume from a first volume where mold compound can be injected easily and completely surround the device with a relatively thick coating to a second reduced volume in which only a thin encapsulating coating remains.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to a method of encapsulating an electronic workpiece and to a mold for use in the method. In the most basic form of the method of this invention, a mold cavity having a first volume has an electronic workpiece positioned therein. Mold compound is added to the mold cavity to encapsulate the microelectronic device, and the volume of the mold cavity is then reduced to a second volume less than the first volume. The mold compound is then cured and the device removed.
In the preferred method of the invention, the mold has a first volume with a size sufficiently large to permit laminar flow of the mold compound around substantially all sides of the electronic workpiece during the step of adding mold compound to the mold cavity. The second volume has a size less than the size necessary to permit such laminar flow. This allows the mold to produce very thin coatings of a thickness less than would otherwise be possible by conventional transfer molding techniques.
In the most highly preferred method of the invention, the mold cavity is adapted to receive a substantially planar electronic workpiece and includes a pair of opposed mold surfaces on opposite sides of the device. The volume of the mold cavity is reduced to the second volume by reducing the distance between the pair of opposed mold surfaces of the mold cavity.
The volume is reduced in one aspect of the method of this invention by providing a tapered clamp cavity defined between two opposed clamp plates having inclined ramp surfaces. The mold is held between the opposed clamp plates and is moved deeper into the tapered clamp cavity to reduce the distance between opposed mold surfaces of the mold cavity.
The mold compound used in connection with this method is typically a thermosetting plastic. In accordance with the method, the mold compound includes a filler. The filler is typically silica, but other fillers can be used to enhance thermal, electrical or mechanical properties of the mold compound.
The invention is particularly suitable for encapsulating microelectronic devices, but is also suitable for encapsulating other thin electronic workpieces, including printed circuits, various types of electronic components, microcircuits and the like.
The invention also includes the mold used in connection with the method described above. The mold includes a mold cavity having a volume defined by a plurality of mold surfaces, a first one of the mold surfaces being movable to reduce the volume of the mold cavity from a first volume to a smaller second volume. The mold cavity is openable to receive a microelectronic device, and an inlet communicates with the mold cavity for adding mold compound. The mold surfaces are sufficiently far from the electronic workpiece when the mold is in the first volume configuration to allow mold compound to be added to the mold cavity without forming voids.